1902. 



FORESTRY AND IRRIGATION. 



i6r 



FIG. 2. SECTIONAL VIEW OF THK EFFECT OF A CONTRACTION FROST CHECK. 



the intense cold. The timber inside, of 

 course, contracts also just as it receives 

 the effect of the cold. Being protected 

 by the outside layer, it contracts much 

 less rapidly. 



The results of this more rapid con- 

 traction of the outer layers of the tree 

 are in a general sense the same as when 

 a machinist or blacksmith attempts to 

 shrink a heated iron band upon a cold, 

 solid shaft. If the band is large enough 

 to go on easily when heated, it will con- 

 tract when cold so as to become prac- 

 tically a part of the shaft ; but if the 

 band is tight when placed on the shaft, 

 the solid shaft being too strong to allow 

 the band to shrink or contract beyond a 

 certain degree, the result is that the band 

 is rent in twain by its own force of con- 

 traction. 



This well-known principle in physics 

 applies to the shrinking of wood under 

 the influence of frost, for as the outside 

 layer of the tree becomes more and more 



contracted by the influence of intense 

 cold, it closes more closely on the warmer 

 wood inside, and as that portion of the 

 timber is being contracted much more 

 slowly, it offers more and more resist- 

 ance to the sudden and rapid contraction 

 of the outside rim. The result can 

 easily be foreseen : the timber outside 

 being unable to withstand the strain, 

 splits or checks with a crack like a 

 pistol shot for a distance up and down 

 the tree sufficient to relieve this strain 

 ( Fig. 2 ) . Some checks extend the whole 

 length of a tree. Such checks very 

 rarely solidify or grow together again, 

 but are kept open by the action of the 

 wind and the effect of repeated expan- 

 sion and contraction caused b}' extremes 

 of temperature. 



As stated above, the effect of the sud- 

 den expansion of a tree caused by a 

 rapid change from a low temperature to 

 a higher is exactly the opposite from the 

 contraction, but with no less serious 



